Method of detecting elementary particles



Dec. 19, 1950 KUAN-HAN SUN METHOD OF DETECTING ELEMENTARY PARTICLESFiled June 19, 1947 D Neu iron Source l Mechanical Caun fer F'luorescenfScreen INVENTOR Kuarl-Han Sun.

v ATTORNEY Patented Dec. 19, 1950 TED f TATEs TENT osmos METHODGFZDETEGTING .ELEMENQARY PARTICLES .KuaneHan Sun, 'Eittsburgh, Pa.,

assignor to Application-aiunedfi, 1947, Serial No.'755,636

2 Claims. .1

-.My .invention relates to the .detectionof elementary particles, andithas particularrelation to the detection -.of elementary particles .of.the neutron type.

,By ielementary .particles, .1 mean atomic or nuclear particles, suchiasneutrons, neutral mes- .ons,,protons,. alpha particles,positrons,..electrons, gamma rays and, photons. .Byelementaryparticles-of ,the .neutronrtype, I mean atomic or nuclearparticles. such as neutrons which do. not produce scintillations whenthey impinge .on .a fluorescent thodysuch. as a=screenof.-zinc.sulphide, zinc cadmium sulphide orcalcium tungstate. Byfelementary particles. of .theprotontype,.I.mean ,particles .Which .do.produce ivisible scintillations when they impinge on .a fluorescentbody.

My inventionmelatesto the application to John W. Coltman and.Fitz-I-IughB. Marshall, Serial No. 752,942,. filed June .5, .1947..fIheiColtman. and (Manshallapplic-ation. discloses a system. for.detecting scintillations produced by elementary parholes. This system isparticularlyefiective in detecting particles incident lat .a .high .rate.and also 5 particles which produce scintillations. of low intensity. Itis, howevenuseful only in-thedete-ction of elementary particles of the.protontype. Elementary particles ofvthe neutron ,type, imping- 'ing on..the fluorescent body disclosed 1 by Goltman and-Marshall :would.iail-to-produce :scintillations and wouldnotbe detected.

Itis, accordingly, an object of :myinvention V to provide apparatus for-001l11l7iI1-g elementary particles of the neutron type.

Another object of invention is :to provide .a methodfor countingelementary particles of the 11 neutron type.

=A vfurther obj ect of my invention is to 3 provide apparatus and amethod forcounting scintillations produced :by elementary :particles ofthe neutron type incident at a'high rate.

.A -still further object of my invention is to' provide :apparatus and amethod for detecting elementary particles of the neutron type incidentat -2a low-number rate.

An ancillaryobject ofrmyinvention isto prowide-anovelfiuorescentmaterial.

Another ancillary object io'fzmy zinventi'oniis'to providea Geigercounterparticularly suitableifor ithe detection of ;particles zo'ftheineutrcn ttyne.

:Still another ancillary object of .mytinvention 'is.tosprovideaafiigereounterwhich shall operate withhighefiiciency to detectelementaryuparticles f theneutrontype.

.A;.further:anci1lary object-of my invention is 'to :provide apparatus.for-efiicientlyl detectin elementary particles.

:In accordance :withuny invention, -I provide a :fluorescent :materialwhich has :mixed therein,

chemicallytconipouncledrtherewithionhas disposed .adjacent' theretoa'material which emits. elemen- .ta y particles ohtheprotontypewhemelementary particles of the neutron type impingetherecn. Forexample, the fluorescent material inaccordance with my invention may becomposed of a mixture of ap'hosphor, suchaszinc'sulphide, or calciumtungstate and a material such as boron 01' its compound, or gadoliniumor its compounds, which emits elementary particles of the proton :type:when elementary particles of the .neutron type impinge :th'ereon..i'Neutron me'active .fluores- .centzmaterials, suchaszincih0rate(ZnBsOi). ,ccad- :mium loorate (CCU-31204) Jor other'boratesiarealsorwitliin the-s-copenfzmy invention.

zA'b'ody of suchreactive Ifluorescent materials isinvaccordanceawithrmyinventionpositioned inathe region Where'theelementaryfparticles of the neurtrontype-are to be detected. "When these'parzti'c'lesimpinge onthe hody,xelementary particles of :the'iprotontype are emitted iby the boron tor :otherreactivel substancerincludedin.it. The .ele- ,mentary particles :of "the -.proton type cause the bodytoiscintillatezand therresultant scintillations are gathered andprojected r onto the :collecting electrode :ofa'phcto-zmultiplier. Theoutput of the photo-multiplier ris :connected to circuits in rthesamemannersas'inithe Coltmanand Marshall system.

.An -aspect of my invention arises from the realization that :thereactive fluorescent tbody zshould becompactandtherefore,should :besolid orfliquidrratherathan gaseous. To:illustrate this thesis,1et:us:assume,:for example, that tlretneutron reactive element whichconverts elementary particles ofithemeutrontype into. elementary :particles 'ofth'e'protontype is the typicalzelement .B ,1that'is,thoronhaving anatomic weight :10. If this ireactive element weretinsgaseousrform, a gas Zincluiiing Tthe element, tfor example, :iooronfluoride QBFa) swouldibe included :in ;an;ioniza- 'ti'on tchamher:Zadjacent ithe phosphor. that :to

be the thickness of a layer of gas which would be required to absorbevery neutron impinging on the gas. to is a measure of the diameter ofthe gas counter of the prior art. The theoretical value of to may bedetermined from the equation in which,

a=capture cross-section for neutrons of the absorbing substance j=weightfraction of absorbing substance in absorber 'W=atomic weight ofabsorbing substance N=Avogadros number d=density or apparent density ofabsorbing substance For BFs,

a=740 barns d=0.00282 gram per cubic centimeter to=5.4 centimeters Theabove calculation is theoretical and yields a value of the thickness ofabsorber which is lower than the required value. In actual fact, theabsorber should have a thickness of the order of seven centimeters, thatis, approximately 3 inches or even thicker. A 3-inch body is excessivelylarge for convenient application.

Let us now assume that a, solid detecting'material such as boron oxide(B203) is utilized. Let us also assume that a fluorescent body iscomposed of 50 per cent boron oxide and 50 per cent phosphor (such aszinc sulphide). Under such circumstances,

j=0.155 and 11:15 grams per centimeter cubed.

Now;

' to=0.012 centimeter.

Accordingly, a very thin layer of solid detecting material is requiredto absorb every elementary particle of the neutron type. A body having athickness of from one to five millimeters serves satisfactorily asreactive fluorescent body. With a small body of this type disposed so asto receive elementary particles of the neutron type and with efficientgathering means for the resulting scintillations, the incidence of suchparticles may be counted even if it is at a high rate with apparatus inaccordance with my invention. With the same apparatus elementaryparticles of the neutron type having low energy content may be detected.

Based on the concept illustrated by the above calculations, I provide,in accordance with another aspect of my invention, a Geiger counter inwhich a thin layer of reactive fluorescent material (for example, amixture of zinc sulphide and boric acid (113303)) and a photo-sensitivematerial are interposed between the ionizable medium and the source ofradiation. Such a Geiger counter may be small since it does not requirea deep layer of gas. The particles of the neutron type are nowcompletely absorbed and converted into scintillations by the reactivefluorescent layer. The photo-sensitive material converts thescintillations into electrons which are absorbed and effectively ionizea thin layer of gas.

The novel features that I consider character,- istic of my invention areset forth with particu larity in the appended claims. My inventionitself, however, both as to its organization and its method ofoperation, together with additional objects and advantages thereof, willbest be understood from the following description of speciflcembodiments when read in connection with the accompanying drawing, inwhich:

Figure 1 is a circuit diagram showing an embodiment of my invention, and

Fig. 2 is a view in section of a Geiger counter in accordance with myinvention.

The apparatus shown in Fig. 1 comprises a reactive fluorescent body 3 onwhich elementary particles of the neutron type are projected from asource 5 for example. A screen (not shown), such as lead slab, may beinterposed between the source 5 and the body 3 to filter out particlesof the proton type; The body 3 includes, either mixed or chemicallycompounded therewith, a phosphor (zinc sulphide, zinc cadmium sulphide,calcium tungstate) and an element, such as boron, which reacts withelementary particles of neutron type to produce elementary particles ofthe proton type. About the fluorescent body 3 a reflector 1 is disposed.When the elementary particles of the proton type are released in thebody 3, scintillations are produced. The scintillations are gathered bythe reflector i and projected on the collecting electrode 9 of aphotomultiplier I (an RCA-93l-A for example).

The optical device l for gathering thescintillations may be of anycurved type. For example, it may be a reflector in the form of a zone ofa paraboloid, or of a sphere or in the form of a section of a circularcylinder or paraboloid of revolution. Under certain circumstances,curved lenses may also be utilized. Any reference hereinafter to areflector or optical device shall be taken to mean an optical system orcomponent for light gathering purpose. The reflector 7 may be composedof any suitable sheet metal properly finished, such as aluminum orstainless steel sheet or chromium plated aluminum or steel sheet. Sinceneutrons transpass such sheet without appreciable loss in number, thereflector E need not be perforated to transmit the particles of theneutron type. The reflector l is thus highly efficient.

The fluorescent body 3 and the collecting electrode 9 of thephoto-multiplier H are disposed at conjugate foci of the reflector 'l.Scintillations produced at the body are reflected to the collectingelectrode and produce a substantial flow of electron current at itsoutput electrode IS. The output electrode is is connected to the grid l5of a tube I! connected in a cathode follower circuit. The outputimpedance is of the cathode follower is connected between the' grid 2|and cathode 23 of an amplifier 25 and the output terminal of the latteris coupled to the grid 2! of a second amplifier 29. The parameters ofthe cathode follower and amplifier circuits are such that the over-allamplifi r system has a video pass band in excess of one megacycle(response curve is in excess of one megacyc.e wide 6 db from maximum).

The output of the video amplifier ll-Z 5-29 is connected to a scalingcircuit 3! of the type 'described in detail in the Coltman and Marshallapplication. The bias of the scaling circuit is so set that it does notrespond to the dark current variations of the photo-multiplier, but doesrespond to the current pulses produced when the light fromscintillations originating at the body 3 impinge on the photo-multiplierelectrode 9. A counter 33is operated. from the scaling circuit 3|. Witha. high ratio scaling circuit 3! elementary particles of the neutrontype incident at rate as high as 100,000 to 1,000,000 for record may bedetected.

The Geiger counter shown in Fig. 2 comprises a hollow vacuumtightcylinder 35 of glass within which an ionizable gas at a low pressure isdisposed. Coaxial with the cylinder 35, a conducting rod Bl which servesas an anode is provided. On the inner surface of the glass cylinder 35,a thin composite layer is provided. The outer face 4! of the compositelayer just under the glass 35 is composed of a thin transparentconducting material and serves as a cathode. The inner face 39 of thecomposite layer is composed of a photo-sensitive compound such ascaesium anti-.

mom'de (SbCSs). The inner face 39 of the composite layer emits electronswhen light impinges thereon. Around the out-er surface of the glasscylinder 3-5 there is a fluorescent screen 43 which reacts withelementary particles of the neutron This screen may be composed of zincsulfide alone. For particles of the proton type, it need not be neutronreactive. For detecting particles of the neutron type a screen composedof a mixture of zinc sulphide (ZnS) and boric acid (HsBOs) in equalproportions by weight may be used. The thickness of the fluorescentscreen 43 should be of the order of one to five millimeters. Thefluorescent screen 43 is encircled by a cylindrical reflecting cylinder45 of sheet metal, such as aluminum or chromiumplated steel, forexample. The reflector may be perforated in one region in countersaccording to my invention, for detecting elementary particles of theproton type.

Elementary particles of the neutron type penetrate the reflector 45 andimpinge on the reactive fluorescent screen 43. In the screen, theyrelease elementary particles of the proton type which, in turn, producescintillations. The direct light from the scintillations and the lightfrom the scintillations repeatedly reflected from the cylindricalreflector 45 impinge on the photosensitive surface 39. The latter emitselectrons which ionize the gas. A high potential (source not shown) isimpressed between the outer face of the composite layer 4! and the anode31, and when the gas is ionized, current flows between the compositelayer 39, 4! and the anode, indicating that a particle of th neutrontype has impinged on the fluorescent screen 43.

The Geiger counter, in accordance with my invention, is shown as acircularly cylindrical structure. My invention may be practiced withstructures of other types such as are available in the tube art. Forexample, the electrodes 31 and 4|, fluorescent screen 43 and reflector45 may have the flattened elliptic form commonly used in vacuum tubes.The cathode and the photosensitive surface may also be interchanged alorproton type.

though in such a modified Geiger counter the electrons would have topass through the cathode into the gas and would lose energy. Within thebroader aspects of my invention also falls a Geiger counter in which thecathode and photosensitive element are one.

Although I have shown and described a certain specific embodiment of myinvention, I am fully aware that many modifications thereof are possible. My invention, therefore, is not to be restricted except asinsofar as necessitated by the prior art and by the spirit of theappended claims.

I claim as my invention:

1. A detector for elementary particles of the neutron type comprising adetecting material sensitive to said elementary particles of the neutrontype and capable of emitting elementary particles of the proton type, aphosphor capable of emitting light when said elementary particles of theproton type are incident thereon as a result thereof, means producing asubstantial dark current for amplifying the pulse of energy emitted bysaid phosphor, and a discriminator connected to the output of said meansfor amplifying for suppressing pulses under a predetermined amplitude.

2. A detector for elementary particles of the neutron type comprising adetecting material sen sitive to said elementary particles of theneutron type and capable of emitting elementary particles of the protontype, a phosphor capable of emitting light when said elementaryparticles of the proton type are incident thereon as a result thereof,said detecting material and said phosphor being substantiallynon-responsive to light, means producing a substantial dark current foramplifying the pulse of energy emitted by said phosphor, and adiscriminator connected to the output of said means for amplifying forsuppressing pulses under a predetermined amplitude.

KUAN-HAN SUN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,030,440 Fritze et al. Feb. 11,1936 2,220,509 Brons Nov. 5, 1940 2,272,375 Kallmann et al. Feb. 10,1942 2,288,717 Kallmann et a1 July 7, 1942 2,288,718 Kallmann et al.July 7, 1942 2,344,042 Kallmann et al Mar. 14, 1944 2,351,028 FearonJune 13, 1944 2,401,288 Morgan et al. May 28, 1946 OTHER REFERENCESCurran et al.: Atomic Energy Commission Document, MDDC, 1296, Nov. 17,1944, 4 pp.

